This invention concerns a hoist which receives a rotational input and applies tension to a pair of opposed flexible tension members with very high mechanical advantage. Hoists of this type include a screw gear which provides a positive hold of the applied tension force when the screw gear is not being driven.
Screw gear hoists are known. The applicant's own U.S. Pat. No. 5,971,178 shows a hoist of this type employing link-and-pin type chains (roller chains) drawn by a load nut or nut gear which is translated by a load screw or screw gear having a connection for receiving rotary input from a pneumatically powered impact tool or from a hand-powered tool.
Hoists of this type, with high mechanical advantage, can be used for lifting or repositioning a heavy load, for fine positional adjustment, or for load binding. Often they can be used in combination with an overhead electric hoist, secured to the bottom hook of such a hoist. The electric hoist lacks precision, and use of the screw hoist enables very precise final adjustment of position of a heavy load. For example, the hoist shown in the above patent can be designed to make an adjustment of about 0.001 inch with input rotation on the order of about one degree.
The term “hoist” as used herein and commonly in the industry, applies whether a load is moved vertically, horizontally or otherwise, or is not moved at all but is only subjected to tension. Improved hoists of this general type, using flexible tension members different from the roller chain disclosed in U.S. Pat. No. 5,971,178, and with guide members to maintain position of the tension members in the housing, form the subject matter of the invention described below.